It is known that photopolymerization of ethylenically unsaturated monomers can be initiated by exposure of the monomers to various sources of radiation. For example, methyl acrylate on long standing in sunlight transforms into a transparent, hardened mass. However, use of sunlight or sources of comparable energy to photopolymerize monomers or combinations of monomers, oligomers, etc., is not practical because of the very slow rate of polymerization.
Photopolymerization with a beam of wave energy such as ultraviolet light is also well known. Typically in carrying out such photopolymerization, a vinyl monomer is formulated with a light-sensitive photopolymerization catalyst (also referred to as a photoinitiator or photosensitizer). When the mixture is subjected to ultraviolet light, free radicals are produced which aid in effecting the polymerization.
Traditionally, developments in the photopolymerization art have been in the area of improving coating techniques or obtaining improved coatings, examples being reduction in reaction time, or achieving higher molecular weight polymers. Little attention has been paid to formulating coatings suitable for specialized applications such as food associated applications; i.e., the inside of can bodies. Such formulations have to meet stringent FDA requirements; and the additives heretofore employed, such as catalysts, have been selected on the basis of coating efficacy rather than on the basis of meeting such requirements.
Certain organic dyes are known to act as photosensitizers, some of which are food dyes approved by the FDA. One such dye, rose bengale (phthalein), designated by the Food and Drug Administration as FD&C red No. 3, is described in a number of patents, for instance, U.S. Pat. No. 3,615,452, by Edward J. Cerwonka. In the process of this patent, the dye is used in combination with a toluene diazonium compound; and no mention is made in the patent of suitability of the coating formulation for food associated uses. An earlier Oster U.S. Pat. No. 2,875,047 also makes reference to this dye but in combination with a reducing agent such as stannous chloride or ascorbic acid. Again no mention is made of suitability for food associated applications.
Another U.S. Pat. No. 3,418,118 to Glenn Anthony Thommes et al. also contains a reference to rose bengale as a suitable photoreducible dye. In this patent, the dye is described as part of a two-component initiating system of which the other component is described as a radical-producing agent, one such agent being triethanolamine. As with the above patents, the efficacy of the photoinitiator system for food associated applications is not indicated.
A patent to Osborn et al., U.S. Pat. No. 3,759,807 describes a two component photoinitiator which comprises an aryl ketone photosensitizer and an organic amine activator such as a tertiary alkanolamine. This patent is of interest in its listing of a large number of organic amines useful as activators. The disclosure of the Osborne et al. patent is incorporated by reference herein.